This paper examines sex preferences for children in Vietnam using the mother birth cohort from 1972 to 1993. We specify the full set for sex–birth order composition of existing children using a split-population model. The model relaxes the assumption in conventional hazard models that all mothers would eventually have another child. Our results indicate a preference in these families for one male offspring. In addition, we observe a mixed sex preference at the fourth birth order. We also establish that a one-square-meter increase in household living area, as a proxy for the increment in household wealth, is associated with a 0.997 times lower preference for a son for the second childbirth, but a 1.015 and 1.014 times higher preference for a son for the third and fourth childbirths, respectively. As long as two-child families dominate the population structure, these results suggest that the importance placed on economic reasons for a son preference will gradually weaken as economic development proceeds in Vietnam.